Proficiency test of plasma free and total metanephrines: report from a study group

Abstract

Keywords: pilot study; plasma free metanephrines; plasma total metanephrines; proficiency testing.The diagnosis of pheochromocytoma is based on the quantitation of O-methylated metabolites of catecholamines (1, 2). Several studies have reported high sensitivity of free metanephrines in plasma for assessing the presence of tumour (1). Measurement of plasma total metanephrines (plasma freeqplasma sulphoconjugated metanephrines) has also been proposed as a diagnostically sensitive assay (3). Therefore, an increasing number of laboratories have developed *Corresponding author: Dr. Eric Grouzmann, PhD, Division de Pharmacologie et Toxicologie Cliniques, University Hospital of Lausanne, Lausanne, Switzerland Phone: q41-21-3140741, Fax: q41-21-3144266, E-mail: eric.grouzmann@chuv.ch assays for free and total metanephrines using different analytical procedures including immunoassays (4, 5), high-performance liquid chromatography (HPLC) coupled to amperometric or coulometric detection (6) and liquid chromatography-tandem mass spectrometry (LC-MS) methods (7). Each laboratory is supposed to establish its own reference values and to perform quality control measurements (QCs) with each set of analyses. The internal QC samples usually are prepared by each laboratory and should include plasma samples spiked with known concentrations of both metanephrine (MN) and normetanephrine (NMN). Participation in an external quality assessment scheme (EQAS) allows direct comparisons between all laboratories that measure free and total metanephrines. In this context, the variability of the upper reference limit for free metanephrines reported in the literature is relatively large. For NMN the range varies from 0.6 to 1.4 nmol/L, and for MN it varies from 0.3 to 0.85 nmol/L (1, 2, 8). However, the upper reference limits for total metanephrines are better defined (3, 9, 10). The analytical procedure for the quantitation of metanephrines in plasma is not the only reason for this variability. The characteristics of the control population may also differ between different studies. The absence of a universal calibrator that could be used by all laboratories performing such measurements encouraged us to establish an EQA program for plasma free and total metanephrines. A similar program already exists for metanephrines and catecholamines in urine. The purpose of this letter is to present the results of a pilot study aimed at implementing a proficiency survey for all laboratories measuring plasma free and/or total metanephrines.We selected 10 centres that routinely measure free and/or total plasma metanephrines for diagnosis of pheochromocytoma. Three analytical methods were used. Immunoassays (1 participant), HPLC with electrochemical detection (ECD) (8 participants) including amperometry, coulometry, coularray and LC-MS (1 participant). All but one laboratory, which used an RIA commercial kit method, used in-house methods. Calibrators and internal quality control samples were manufactured using spiked serum pools with standard dilutions of Sigma...

Influence of various confounding variables and storage conditions on metanephrine and normetanephrine levels in plasma

“…For normetanephrine, intra‐assay and inter‐assay coefficients of variation of 15% (for 155·5 ± 5·4 ng/l) and 7–17% (7% for 882·4 ± 15·1 ng/l, 17% for 90·3 ± 3·8 ng/l), respectively, were found. Whereas a recent publication showed that the immunoassay from LDN had inter‐assay coefficients of variation of up to 31%, 14 the generation of assays provided for the current study demonstrated better precision. Apparently, for unknown reasons, specific production cycles of an older assay generation had larger variability especially in the lower range.…”

Diagnostic accuracy of free and total metanephrines in plasma and fractionated metanephrines in urine of patients with pheochromocytoma

“…Interestingly, our healthy subjects had lower plasma norepinephrine and total metanephrines, plasma free and urinary metanephrine than the controls (average differences 24-43%, P!0.05). This underscores the importance of establishing local reference values for such biochemical parameters until inter-laboratory standardization is achieved (19).…”

Measurements of plasma metanephrines by immunoassay vs liquid chromatography with tandem mass spectrometry for diagnosis of pheochromocytoma

“…The negative bias of EIA measurements of plasma MNs was initially suggested from results of an inter-laboratory quality assurance program that utilized pooled lyophilized plasma spiked with commercially available racemic enantiomers of MNs in the first series (11), and plasma spiked with natural L-enantiomers derived from urine was used in a subsequent series (12). The distinction is important, as the natural L-enantiomers of MN are the forms targeted by immunoassays (5).…”

“…However, none of these studies compared the immunoassay with either LC-ECD or LC-MS/MS. First reports from inter-laboratory proficiency programs suggest that immunoassays may be less precise and accurate than both other methods (11,12). Suggestions that immunoassays show a negative bias, leading to potential missed diagnoses, were further suggested by two other studies (13,14).…”